1. Field of the Invention
The present invention relates in general to a grinding process and a grinding apparatus for producing and finishing a surface of a workpiece with a high accuracy.
2. Discussion of Related Art
There is known, as a kind of grinding apparatus, a rotary-type vertical spindle surface grinder including: a work rotating device operable to rotate a workpiece about its axis parallel to a vertical direction; a grinding-tool rotating device operable to rotate the grinding tool about its axis parallel to the vertical direction for grinding a surface of the workpiece; a supporting device which supports the grinding-tool rotating device such that the grinding-tool rotating device is movable in a direction parallel to the axis of the grinding tool; and a grinding-tool moving device operable to move the grinding tool toward the workpiece in the direction parallel to the axis of the grinding tool. As an example of the rotary-type vertical spindle surface grinder, JP-H08-276349A (publication of unexamined Japanese Patent Application laid open in 1996) discloses a surface grinder in which a feed movement of the grinding tool or workpiece is controllable with accuracy in order of submicrons or nanomicrons.
In the rotary-type vertical spindle surface grinder disclosed in the publication of the Japanese Patent Application, a position of the grinding tool (grinding wheel) is constantly detected, and the detected position of the grinding tool is input to a controller, so that a hydraulic cylinder as the grinding-tool moving device is controlled by the controller, on the basis of the detected position of the grinding tool. That is, the disclosed grinder is equipped with a feedback control system in which the hydraulic cylinder is controlled in a closed-loop manner, whereby the workpiece can be grounded accurately by a desired amount of depth of cut of the grinding tool into the workpiece.
Like in the above-described disclosed grinder, in a conventional rotary-type vertical spindle surface grinder, it is common that the grinding tool is moved at a constant feed rate in an infeed direction (that increases the depth of cut) until the workpiece has a desired dimension or depth of cut. In such a machining method, the grinding tool is moved in the infeed direction to increase the depth of cut until the workpiece is given the desired dimension, irrespective of a grinding load which acts on the grinding tool and which is represented by a pressing force or a drive current. Therefore, a machining operation according to the conventional method can not be necessarily carried out with a sufficiently high efficiency, and can not necessarily provide a sufficiently high quality of the machined workpiece. For example, in the conventional machining operation which is carried out at a constant feed rate such that the workpiece has a desired dimension, the grinding tool could be forcedly moved at a constant feed rate in the infeed direction even in event of reduction in a grinding capacity of the tool due to clogging of its grinding surface, thereby causing glazing on the grinding surface of the tool and consequently making impossible to provide a satisfactory quality of the machined workpiece. The glazing on the grinding surface might be avoided by reducing the feed rate or reducing an amount of stock removal per a unit of time. However, such a reduction causes the workpiece to be ground with an insufficient pressing force, namely, with a low machining efficiency.